In a curved stairlift installation the angle of the stairlift rail varies with respect to a horizontal plane. As the stairlift carriage moves over a transition bend (a bend in a vertical plane) the chair mounted on that carriage, must be kept level. In the embodiment of stairlift described in our European Patent No. 0 738 232 the chair is pivotally mounted on the carriage and a chair levelling motor operates to keep the chair level as the angle of the rail varies.
An arrangement of the type shown in EP 0 738 232 gives rise to a concern that, in the event of failure of the chair levelling motor and/or transmission, the chair could rotate in an uncontrolled manner relative to the carriage. Obviously, in the event of such a failure, a person seated in the chair could be injured. Mindful of the possibility of such failures, EP 0 738 232 also describes the use of a back-up safety arrangement in which a pair of mercury switches trigger the release of a locking pin when the chair angle reaches a predetermined off-level upper limit on either side of a central, level, position. The locking pin, in turn, extends into a locating aperture in the chair interface so as to lock the position of the chair relative to the carriage. The locking mechanism also triggers the main safety circuit which brings the carriage to a halt.
Since the filing of EP 0 738 278 it has become commonplace to substitute a single analogue tilt sensor for the mercury switches however, in common with
the mercury switches, a tilt sensor only gives an output signal when the off-level limit is reached.
This off-level limit is typically 5° however, because the chair may have built up considerable momentum before the failure is detected, the locking mechanism triggered, and the locking mechanism engages, the applicable standards prescribe that that chair must be brought to a halt within 15° of vertical.
This problem is addressed in our International Patent Application WO2008/142372. This patent application describes an arrangement in which movement of the levelling drive, and rotational movement of the chair are separately monitored using encoders. In the event that the encoder readings vary, a spring loaded pin is released by a solenoid to engage in a slot formed in an indexing ring attached to the chair. The indexing ring is provided with a series of slots with the intention that, should the locking pin be released, it will engage in that slot which, when the pin is released, is located closest to the pin.
Whilst this solution offers significant improvements over that which preceded it, it still presents the following issues:    1. Response time in the event of failure is critical. Any failure of the levelling system must be detected, and the locking pin released and engaged in a slot in the indexing ring, within 15° of movement of the chair from the vertical. This has proved difficult to achieve on a consistent basis.    2. The slow response time has, in some instances, been due to backlash inaccuracies in one or both of the encoders, which inaccuracies are exacerbated by wear.    3. When released by the solenoid, the locking pin can skip the closest slot in the indexing ring and only engage in the second or third available slot. This makes the response time very unpredictable. More slots could be added to address this problem but, for a given diameter of indexing ring, this would then necessitate reducing the widths of the slots and diameter of the locking pin. The consequence of this would be to leave the locking pin susceptible to failure when subjected to the significant shock loads imposed when arresting the rotating chair. Similarly, adding slots to the indexing ring would reduce the material between adjacent slots, thus weakening the ring itself. In compact arrangements such as a stairlift carriage, significantly increasing the diameter of the indexing ring is not an option.    4. There is a preference, in the stairlift field, for safety facilities which are mechanically based rather than electrical/electronic based.
It is an object of the invention to provide a safety method and apparatus for a stairlift which will go at least some way in addressing the aforementioned problems; or which will at least provide a novel and useful choice.